This invention relates to a process for producing high aspect ratio flakes that can be used for both functional and decorative applications. The flakes can be metal, metal compounds, non-metal or clear flakes. Functional applications of the flakes include uses in protective coatings in which the flakes can add a certain level of rigidity to produce certain desired properties of the finished coating, or in which the flake layer can be used to screen out light of certain wave lengths to protect an underlying pigmented layer. Reflective metal flakes are useful in a variety of optical or decorative applications, including inks, paints or coatings. Other uses of the flakes include microwave and electrostatic applications.
Conventional aluminum flake is manufactured in a ball mill containing steel balls, aluminum metal, mineral spirits, and a fatty acid usually stearic or oleic. The steel balls flatten the aluminum and break it into flakes. When the ball milling is complete the slurry is passed through a mesh screen for particle sizing. Flakes too large to pass through the screen are returned to the ball mill for further processing. Flake of the proper size is passed through the screen and introduced to a filter press where excess solvent is separated from the flake. The filter cake is then let down with additional solvent. Such conventional aluminum flake typically has a particle size from about 2 to about 200 microns and a particle thickness from about 0.1 to about 2.0 microns. These flakes are characterized by high diffuse reflectance, low specular reflectance, rough irregular flake micro surface, and a relatively low aspect ratio.
Another process for making metal flakes is a process of Avery Dennison Corporation for making flakes sold under the designation Metalure. In this process both sides of a polyester carrier are gravure coated with a solvent-based resin solution. The dried coated web is then transported to a metallizing facility where both sides of the coated sheet are metallized by a thin film of vapor deposited aluminum. The sheet with the thin metal film is then returned to the coating facility where both sides of the aluminum are coated with a second film of the solvent-based resin solution. The dried coated/metal sheet is then transported again to the metallizing facility to apply a second film of vapor deposited aluminum to both sides of the sheet. The resulting multi-layer sheet is then transported for further processing to a facility where the coatings are stripped from the carrier in a solvent such as acetone. The stripping operation breaks the continuous layer into particles contained in a slurry. The solvent dissolves the polymer out from between the metal layers in the slurry. The slurry is then subjected to sonic treatment and centrifuging to remove the solvent and the dissolved coating, leaving a cake of concentrated aluminum flakes approximately 65% solids. The cake is then let down in a suitable vehicle and further sized by homogenizing into flakes of controlled size for use in inks, paints, and coatings.
Metal flakes produced by this process for use in printable applications such as inks are characterized by a particle size from about 4 to 12 microns and a thickness from about 150 to about 250 angstroms. Coatings made from these flakes have a high specular reflectance and a low diffuse reflectance. The flakes have a smooth mirror-like surface and a high aspect ratio. The coatings also have a high level of coverage per pound of flake applied when compared with metal flakes produced by other processes.
Flakes also are produced in a polymer/metal vacuum deposition process in which thin layers of vapor deposited aluminum are formed on a thin plastic carrier sheet such as polyester or polypropylene, with intervening layers of cross-linked polymers between the vapor deposited aluminum layers. The cross-linked polymer layers are typically a polymerized acrylate deposited in the form of a vaporized acrylate monomer. The multi-layer sheet material is ground into multi-layer flakes useful for their optical properties. Coatings produced from such multi-layer flakes tend to have a high diffuse reflectance and a low specular reflectance. The flakes have a low aspect ratio and undesired low opacity when made into an ink. The materials resulting from this process have multiple layers that cannot be separated into individual layers to form flakes having a high aspect ratio and a high level of micro-surface smoothness (brightness).
An objective of the present invention is to reduce the number of manufacturing steps and the resulting cost of making high aspect ratio, highly reflective metal flakes.
The present invention comprises a flake forming process in which a multi-layer film is applied either to a thin, flexible polymeric carrier sheet such as polyester, or to a polished metal casting surface such as a metal drum. In either instance the process is carried out in a vacuum deposition chamber. In one embodiment, in which the multi-layer film is applied to a polyester carrier sheet (PET), the polyester film can be thinner than 50 gauge and the film can be pre-treated with smoothing and release layers. The vacuum chamber is equipped with multiple coating and deposition sources. Organic materials can be deposited by liquid applicators or spray equipment and can be UV or EB cured. The deposition sources can be vaporization at elevated temperatures caused by heating by induction or EB. Air is evacuated from the chamber and the PET film is unwound past the coating and deposition sources while kept in contact with a cooling drum. Alternating layers of materials can be applied to the moving PET web. One example is a solvent-soluble polymer organic or inorganic material (about 200 to about 400 angstroms), followed by a layer of metal such as aluminum (150 to 250 angstroms), followed by another layer of the solvent-soluble coating. Other metals or inorganic compounds may be substituted for the aluminum. By reversing the web path and inactivating the second coating source and then repeating the first step, many layers can be applied to the PET without breaking the vacuum, which can increase productivity. Additional protective layers can be deposited on each side of the aluminum layers by adding two additional deposition sources between the coating and metal deposition sources. The multi-layered coated PET is introduced into a solvent or water stripping process to remove the sandwich from the PET. The solvent or water is then centrifuged to produce a cake of concentrated flake.
In an alternative embodiment, the same coating and deposition techniques are used to apply alternating layers directly to a release coated cooling drum contained in the vacuum deposition chamber. The drum is rotated past the coating and deposition sources to build up a multi-layer sandwich sheet that is later removed from the drum. The multi-layer sheet is then introduced directly into a solvent with or without suitable agitation to produce flakes; or it can be ground to rough flakes which can also be air-milled to further reduce particle size, and then introduced into a solvent or water slurry to allow the remaining layers to be separated. The solvent or water may be removed by centrifuging to produce a cake of concentrated metal flakes.
The cake of concentrated flakes or the slurry of solvent and flakes then can be let down in a preferred vehicle and further sized and homogenized for final use in inks, paints or coatings.
These and other aspects of the invention will be more fully understood by referring to the following detailed description and the accompanying drawings.